Kristen Garofali

Broadly, my research interests are in probing massive star and binary evolution from the X-ray perspective. More specifically, I study high-mass X-ray binaries (HMXBs) and supernova remnants (SNRs) (latest SNR paper here) in order to learn more about the life and death cycles of massive stars, and the effects of binaries on massive star evolution. For my thesis, I am focused on probing these populations in the nearby, star-forming spiral M33 using Chandra, Hubble, and XMM-Newton data.

This animation first displays the deep Chandra coverage available for M33 from Tullmann et al. (2011), and next the Local Group Galaxy Survey (Massey et al. 2006) of M33 overlaid with the archival HST fields pertinent to this work (magenta). Cyan crosses are X-ray sources from Tullmann et al. (2011) that fall within the current archival HST coverage, and the white crosses are X- ray sources contained in the upcoming M33 HST legacy survey (white regions; PI: J. Dalcanton). There are 286 X-ray sources covered by 91 HST fields. We expect approximately 40 of the X-ray sources in these fields are HMXBs. The next two frames are zoom-ins on one portion of an RGB (F814W,F606W, blue equivalent) rendered HST ACS field from the disk of M33 with the X-ray error circle (0.7") shown in white, and the position of the potential optical counterpart to a candidate HMXB denoted by the white arrow.

High-mass X-ray binaries (HMXBs) provide an exciting window into the impact of mass transfer on massive star evolution. They are also likely progenitors of gamma-ray bursts and gravitational wave events, and may be an important source of feedback in the early universe. Part of my thesis is focused on the identification and characterization of the HMXB population in M33. By leveraging archival data from both Chandra and the Hubble Space Telescope (HST), we are able to identify HMXB candidates and their potential optical counterparts after careful astrometric alignment (above image). With candidate HMXBs identified, we can then measure their ages to ~5 Myr precision from fits to the color-magnitude diagrams of the surrounding stars, which yield the star formation histories (SFHs) of the surrounding region. This age measurement also yields information about progenitor mass and energetics. Furthermore, we can use a combination of the X-ray and optical characteristics of the HMXBs to constrain physical parameters for some of the HMXBs, including potential periods and companion masses in addition to ages. The distribution of these parameters for the population of HMXBs can be directly compared to models of HMXB production expected from the known SFH of M33. These comparisons put new constraints on prescriptions used by binary population synthesis models which attempt to map the characteristics of the parent population to the resulting HMXBs. Below is an example of the HMXB age recovery method for M33 X-7, and known eclipsing X-ray binary hosting a 90 solar mass star in a 3.5 day orbit around a 15 solar mass black hole. We recover an age of < 6 Myr for this system. In addition, we show the age recovery method for a candidate HMXB discovered in this work, for which we derive an age
< 20 Myr.

Left: Cumulative star formation history for stars younger than 100 Myr from CMD fitting using MATCH (Dolphin 2002), with the most likely age denoted with the dashed lines (errors in red), and the Monte Carlo derived errors on the cumulative star formation history in grey. Right: The SFH for the 50 pc region surrounding M33 X-7. The most likely age for this HMXB candidate is < 6 Myr. Inset is an image of the optical counterpart from HST (F439W), with the X-ray error circle in cyan.

Left: The HST CMD of stars within 50 pc of this candidate HMXB (cyan star). For reference isochrones from the Padova group are overlaid for ages (from blue to red) of 6, 10, 20, 50, and 100 Myr. Center, Right: Cumulative star formation history for stars younger than 100 Myr from CMD fitting using MATCH (Dolphin 2002). The most likely age for this HMXB candidate is < 20 Myr.